The present invention relates to improvements in anchor devices.
It is known to provide an anchor device based on a self-tapping screw design. Such an anchor device being based on a tapered helix ramp begins to lose its penetrative and anchorage properties as the material (the xe2x80x9chost materialxe2x80x9d) into which it is screwed becomes harder, less elastic and/or fibrous.
A conventional self-tapping screw has a helix coil that protrudes from the body of the screw in the form of fin-like profiles. The fins are predominantly tapered to be substantially sharp at their outer tips. The taper angle of the helix profile fin is usually substantially the same either side of the central longitudinal axis of the fin. The space between adjacent fins are generally of a V to a U shape or a transform between the two. This transformation is likely to be due to the change in the rising angle of a helix profile necessary to gain a given pitch with the changing circumference brought about by the taper. The V or U shape naturally creates a greater total surface per screw perimeter area than would a straight taper measured at the helix outer perimeter.
As suitable as a tapered helix may be for elastic and fibrous host materials, it becomes less suitable as the host material becomes harder, less elastic and/or fibrous.
There are two primary and often interlinked reasons why conventional self-tapping screw design anchors fail. On the one hand failure can occur when unsustainable demands are placed on the inward penetration forces, screw turning forces and screw strength. On the other hand, failure can result when the helix thread path is broken or stripped.
Likely cause and effects involved in failure of a self-tapping screw design anchor include:
The top to bottom cavity spaces and surface areas between helixes become an entrapment for host material. The material must be transformed from its in situ state into a state of elasticity or fluidity which under pressure begins to adhere to the surface area. The fluidity may be due to host material elasticity or movement of broken down, granulated host materials or combinations of the two.
Harder host materials create higher volume displacement resistance to the penetration of the anchor and upward forces act against the helix or thread path. This upward force combines with a twisting and expansive force versus directional motion force relative to the axis of the screw which deforms the host material into a fluid-like moving state. The greater surface area of the entrapping V or U shape causes the host material to adhere to the surface area. The rotary movement of the screw penetration thus demands a sheer between the screw and the host material relative to directional motion and forces.
Forces occurring internally within the host material, screw and host material strength and helix design thus determine the limitation of a self-tapping screw for a particular host material.
Host material without fibre, or at a force beyond adequate fibre support allows the helix encased host material to adhere to the screw and to form a shear line of least resistance that inevitably forms an upward expanding conical shape. Should the screw helix design and strength be greater than the host material under the forces applied then the host material would shear at the upward expanding cone shape defined by the outer perimeter of the helix or helixes. Should the host material be the stronger the forces may be sufficient for the helix to sheer at the body of the screw thus resulting in a stripping of the threaded path. Alternatively, random spaces between the screw body and the helix perimeter the shear line will pass through both helix and host material and strip.
A further cause of screw failure can arise from volume displacement resistance. Harder materials have the strength at volume or distance to xe2x80x9cfloatxe2x80x9d up or resist the penetration of the volume of the anchor device. The directional force of volume displacement resistance both influences the direction of shear and the effect it has on shearing the helix path such that the screwed rotation breaks away a carrot shaped cone and the helix path leading to failure. Elastic and fibrous materials create friction resistance to turning mostly by sideways forces. Experience shows that volume displacement forces in soils and soft fissil is the prime cause of stripping.
An object of the present invention is thus to provide a screw form anchor device which provides better control of the forces that govern what the screw and the host material must endure to function when host materials become stronger, harder, less flexible and less fibrous.
According in one broad aspect of the invention there is provided a screw form anchor device including a body having a manipulating end and a distal end, a screw thread formed by a plurality of tapering angular helix thread forms extending away from said distal end toward said manipulating end, each angular helix thread form having an outward facing surface and, relative to the distal end, a downward facing surface. In one form of the invention an emergent helix thread commences from a point distant from the distal end and extends toward the manipulating end. The emergent helix thread can be continuous or discontinuous.
In the preferred form of the invention the outward facing surface is directed upwardly relative to the distal end. The downward facing surface is substantially normal to a longitudinal axis of symmetry of the body. In the preferred form the distal end of the body is pointed. The emergent helix can be derived from one of said plurality of helix thread forms.
The emergent helix thread preferably has an outward and upward facing peripheral surface for at least part of its length. Preferably the emergent helix thread has an outward and downward facing peripheral surface as the emergent helix thread approaches a terminal end remote from the distal end.
According to one form of the invention one or more of said helix threads of the helix thread form can merge at a point distant from said distal end. Such merging can occur prior to or after the commencement of the emergent helix thread.
One form of the invention has the helix thread form discontinuing at a point after commencement of the emergent helix thread.
The emergent helix thread can be of a tapering or substantially constant peripheral diameter or a combination of both.
In one form of the invention the body is joined at the manipulating end by an extension body which includes a helix thread. A multiplicity of said extension bodies may be provided and joined longitudinally end to end relative to the anchor device.